The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for spectrally-resolved three-dimensional MRI without using frequency-encoding gradients.
Acquiring images with MRI in the presence of metal objects is challenging because of off-resonance, signal loss, and signal pile-up artifacts that occur in the highly inhomogeneous magnetic field environment surrounding the metal object, or in other regions of magnetic field inhomogeneities. Recently, new methods aimed at improving the reliability of MRI in the presence of metal objects have been proposed; however, these methods still have significant limitations.
One method, referred to as MAVRIC, reduces through-plane distortion artifacts, and utilizes a three-dimensional acquisition for many distinct frequency bins that encompass the distribution of frequencies found near the metal object. The signals acquired in these disparate frequency bins for each voxel are summed together to produce the final image.
Another method referred to as SEMAC addressed through-plane distortion artifacts by employing phase-encoding along the slice-encoding direction for each slice select excitation, thereby resolving non-linear slice profiles.
Although the MAVRIC and SEMAC methods approached the problem of performing MRI in the presence of metal objects from different perspectives, they both solved the problem in a similar manner: by utilizing three-dimensional imaging at distinct frequency bins. For this reason, the MAVRIC and SEMAC methods have also been combined into a hybrid method that demonstrated improved imaging capabilities.
Although these methods have demonstrated improved imaging in the presence of metal objects, they still suffer from in-plane spatial distortions along the frequency-encoding dimension. These distortions occur because these methods acquire data during the use of a frequency-encoding gradient, which leads to pile-up artifacts and distortions. While view-angle tilting and Jacobian methods can be used to help reduce in-plane signal loss and pile-up errors near metal objects, errors are unavoidable in voxels where the local gradient inside the voxel exceeds that of the frequency-encoding gradient. Thus, all imaging methods that utilize a frequency-encoding gradient for spatial encoding are limited in their ability to eliminate in-plane signal loss and pile-up.
It would therefore be highly desirable to provide a method for magnetic resonance imaging (“MRI”) in which frequency-encoding-related shift artifacts are not present. Furthermore, it would be desirable to provide such a method that would be capable of measuring local magnetic field inhomogeneities, characterizing different chemical species, and measuring relaxation rates of transverse magnetization, R*2, by way of chemical shift encoding.